I've got the 70-300 ED lens. It is OK for vacation snapshot photos, and if you are not going to make large prints you will find it very nice. It is lightweight, and fairly sharp. If you are going to take "good" photos (for lack of a better descriptor) where you want sharpness all the way to the corner, and good low-light use, you'll find any of the above lacking. For that price range, though, go with the ED lens. The G has a plastic mount, and arguably lesser quality glass. I gave the Sigma a try in the store, and it was OK. I already had a macro, so I passed for the Nikor.
As for the circular polarizer, get the one that fits your largest lens, and use a step-up ring to make it fit your smaller lens. Your kit lens is a 67mm, and the 70-300 is 62, so if you get a 62 -> 67 step-up ring, you can use one polarizer on both, without vignetting problems. get one plenty early before you leave, because the 67mm is an odd size. Get the best polarizer you can talk yourself into spending. You don't want a good lens with a less than perfect piece of glass in front of it. especially when you get better lenses. You don't shoot through house or car windows, do you? Then don't shoot through a $10 piece of glass.
I just shot 320 images at an autocross this morning, almost all with the 18-70 at the long end of the range, and with the 180mm f/2.8. That sucker is a beautiful lens! I also just got back from a week's vacation in Bermuda, and took my 18-70, 70-300, 16mm, 50mm, and 180mm. I used the 16mm for about 10-15% of the images, the 180mm for about 10%, and the 18-70 for everything else. I never event took the 50mm or 70-300 out of the bag.
Look at a monopod for the 70-300! It gets to moving out at 300mm at slow exposures. The feisol is light and compact. You won't even know you've packed it.
Here's my toys, and if you click on the D-70 you can see the accessories.
Here's today's autocross shots:
If you haven't already found them, there is a D-70 forum, and Nikor lens forum at www.DPReview.com. Another is www.Nikonians.org
Hope this helps,